We demonstrate the possibility of reversible pH-controlled sol-gel transition in aqueous solution of associating amphiphilic triblock copolymer poly(styrene-grad-acrylic acid)-b-poly(acrylic acid)-b-poly (styrene-grad-acrylic acid), (PS-grad-PAA)-b-PAA-b-(PS-grad-PAA), synthesized via nitroxidemediated (NM) radical copolymerization. The presence of pH-sensitive co-monomer units of the acrylic acid in the terminal blocks ensures the dynamic nature of the styrene-rich hydrophobic nanodomains which are formed at low pH. At small polymer concentrations the association triggered by lowering the pH gives rise to flower-like micelles stabilized by partially ionized PAA coronae. The pH-controlled association was monitored by DLS-titration and manifested in the evolution of a correlation peak in the SANS spectra. The resulting copolymer aggregates were visualized by TEM, which confirmed the spherical shape of the dense styrene-rich domains. Above the micelle overlap concentration a decrease in pH provokes macroscopic gelation. Here the styrene-reach domains perform as cross-links in the transient network. The pH-triggered sol-to-gel transition is manifested in an abrupt and strong (up to 3 orders of magnitude) increase in the zero-shear viscosity and in a characteristic change in the frequency dependence of the storage and loss moduli. The discovered effect can be used for efficient pH-control of the rheological properties of aqueous solutions.
A principle conception for quasiliving TEMPO-mediated radical copolymerization was advanced. It was concluded that TEMPO-mediated copolymerization of styrene with other monomers (which are not able to polymerize via quasiliving mechanism) may proceed via quasiliving mechanism in two regimess"azeotropic" and "gradient" according to the reactivity ratio of comonomers. The "azeotropic" regime gives an almost linear increase in molecular weight with conversion and provides controlled synthesis of copolymers with a low polydispersity index. "Gradient" copolymerization proceeds via a deadend mechanism with formation of gradient copolymers. A simple kinetic scheme was proposed for "azeotropic" copolymerization. Experimental kinetic data correlating with the advanced concept were obtained for quasiliving copolymerization of styrene with methyl and butyl acrylates, methyl methacrylate, and acrylonitrile ("azeotropic" regime); and for styrene copolymerization with N-vinyl pyrrolidone and vinyl acetate ("gradient" regime).
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